1. Field of the Disclosure
The present disclosure relates to a black toner formulation for use in electrophotography for generating dark images, and more specifically, to a composite black toner formulation that has a low pigment loading, and is capable of exhibiting good darkness and hue, good fusing ability, and improved resistance to fade.
2. Description of the Related Art
Various types of toners have been used for generation of images on image-receiving media using a printing technique, such as electrophotography. Suitable examples of an image-receiving medium include, but are not limited to, paper, plastic, and textile. The technique of electrophotography is broadly used in photocopying machines, laser printers, Light-Emitting Diode (LED) printers, and the like. More specifically, the technique includes transfer of a specific toner to an image-receiving medium with the help of electrostatic charges. In other words, any printing system working on the principles of electrophotography employs an image-receiving medium, which during the course of a printing process gets imbued with variable areas of the electrostatic charges that correspond to an image to be printed. These variable areas of electrostatic charges are responsible to interact with the specific toner that subsequently may be fused and fixed on the image-receiving medium to generate a printed image thereon.
In general, a desirable quantity of a toner is required for printing images of a good quality on an image-receiving medium. Such a desirable quantity may be defined in terms of density, mass per unit area (M/A) of the toner, which is ideally characterized by a number of monolayers of the toner to collectively form a toner layer as applied on the image-receiving medium to form an image thereon. Therefore, controlling the quantity of the toner, and thereby ensuring consistent color reproducibility each time while printing requires a control over thickness of each toner monolayer as applied over the image-receiving medium. Consequently, toner patch sensors have been employed in electrophotography-based printing systems to control thickness of the toner layer applied over the image-receiving medium. The toner patch sensors are capable of monitoring the toner density of unfused images, thereby providing a means to control darkness of the printed images.
However, control of image darkness and the associated intensity of color of a toner, has always been challenging in electrophotography-based printing systems. More specifically, such a problem is prevalent for producing dark images using a black toner. In general, a conventional black toner used for generating black colored fixed or stable images employs a black colorant. A suitable example of the black colorant is carbon black.
However, carbon black has been identified and listed as a carcinogenic agent by governmental authorities of various countries. Accordingly, large-scale use of carbon black has been discouraged in many countries. Further, a conventional black toner that has a high carbon black loading strongly absorbs light in a wavelength of about 200 nanometers (nm) to about 2000 nm. Therefore, the conventional black toner may produce a relatively weak reflected infrared signal for a toner patch sensor that operates at a wavelength near 940 nm (hereinafter referred to as ‘toner patch sensor wavelength’). More specifically, as the toner patch sensor emits and detects light at a wavelength ranging from about 750 nm to about 1000 nm, and more particularly, from about 900 nm to about 1000 nm, strong absorbance at the toner patch sensor wavelength acts to prevent most photons, emitted by the toner patch sensor, from interacting with toner particles that are present in a lower portion of the toner monolayer. This results in a degraded ability to determine and regulate thickness of the toner layer.
Consequently, black toner formulations with reduced carbon black levels have been developed. Such black toner formulations include different types of color pigments and dyes, and exhibit good darkness and hue along with good light fastness with reduced loading (such as 1.5 percent) of carbon black. By using significantly reduced carbon black loadings, toner powder reflectivity of such black toner formulations increases from about 2% to about 6%. Such an increase in toner powder reflectivity enables a toner patch sensor to detect the presence of black dots even on intermediate transfer belts with low reflectivity. A suitable example of such an intermediate transfer belt is one made of ethylene-tetrafluoroethylene copolymer (ETFE). Accordingly, an electrophotographic printer that employs such a black toner formulation is capable of accurately rendering black shade levels on an image-receiving medium. Further, detection of high toner powder reflectivity may prove useful for anti-counterfeiting and document authentication. Furthermore, the reduced carbon black loading has shown to significantly reduce a print defect associated with the transfer of the black toner formulation to an image-receiving medium in humid conditions.
However, such black toner formulations are more expensive to manufacture than conventional black toners due to the use of expensive color pigments, which are used as substitutes for carbon black for generating dark images. Further, the color pigments are required to be dispersed properly while preparing the black toner formulations, and accordingly, such a requirement significantly contributes to the processing cost of the black toner formulations. Furthermore, use of the black toner formulations that include a blend of color pigments poses fusing problems. Specifically, black toner formulations with reduced loading of carbon black require large quantities of the color pigments to be employed therein for adequate tinctorial strength. As used herein, the term “tinctorial strength” may refer to an optical absorbance per unit concentration or loading of a color pigment. Accordingly, a high total pigment loading (for example, total pigment loading of greater than about 9.5%) in a black toner formulation does not allow the black toner formulation to fuse as easily as a conventional black toner. Consequently, high fusing temperatures are required for allowing fusing of the black toner formulation. However, the use of the high fusing temperatures may result in reduction of effective lifetime of various fusing components of a printer, while increasing energy consumption of the printer.
Further, few black toner formulations that include a blend of dyes have also been developed. Suitable examples of such dyes include, but are not limited to, a blue anthraquinone dye and a black indanthrone dye compound. However, most of the dyes in such conventional black toner formulations have low fade resistance. Consequently, the black toner formulations are incapable of exhibiting good darkness and hue over a period of time.
Accordingly, there is a need for a black toner formulation that is characteristic of exhibiting good darkness and hue, good fusing ability, and improved resistance to fade.